The gas generators of this type include the one having a housing made of an aluminum alloy, iron, or steel. Because a housing made of an aluminum alloy is readily molded compared to a housing made of iron or steel, in some housings, an ignitor holding portion for fixing an ignitor is integrally formed at a center of a lower lid, as disclosed in Japanese Unexamined Patent Publication No. Hei 10-081190. In some housings made of iron or steel, for example, an ignitor holding portion (a holder for an ignitor), that is another member, is fixed by welding on a press-formed lower lid, as disclosed in Japanese Unexamined Patent Publication No. Hei 11-078764.
In consideration of environment and safety, there is a tendency to change an azidation gas generating agent in which  sodium azide is used to a nitrogen containing organic-compound gas generating agent.
However, a combustion temperature of the nitrogen containing organic-compound gas generating agent is likely to be high compared to that of the azidation gas generating agent. A higher pressure is required to keep the same level of burning characteristic and a housing made of an aluminum alloy and having a construction of a single cylinder has a limit in heat resistance and pressure resistance.
A housing should have a sealing construction to protect a gas generating agent from deterioration by moisture absorption. If an ignitor holding member is fixed by welding on the housing made of iron or steel, in addition to a welding process, there is a possibility that a problem occurs with regard to reliability in sealing of a welded portion. Therefore, from necessity to keep the reliability, a leak test should be carried out beforehand.
Similarly, in a case that the ignitor holding member is fixed by caulking, it is necessary to add a sealing member such as a packing, to fix by caulking, and to carry out a seal test.
In a case that a length in an axial direction (L) is shorter than a diameter (D) of a gas generator (L/D≦1), a lower lid cannot be so thick as to prevent a modification by a high internal pressure generated at the time of burning of a gas generating agent. If the  lower lid is modified by a pressure generated at the time of burning of the gas generating agent, there may occur a leak of a combustion gas with a damage of a welded portion of an ignitor holding portion and unfixing of a caulking. There is an adverse possibility that the leaked combustion gas does not go through a cooling/filtering member.
It is an object of the present invention to solve various problems described above, specifically, to provide a gas generator suitable for a nitrogen containing organic-compound gas generating agent and excellent in heat resistance, pressure resistance, and sealing.